1. Field of Invention
The present invention relates to preparation of ortho substituted aryl fluorides from corresponding diazonium fluorides. More particularly, the invention relates to a method for thermally decomposing such diazonium fluorides to form ortho substituted aryl fluorides wherein the ortho substituted entity is halogen or trihalomethyl.
2. Discussion of the Prior Art
Fluoroaromatic compounds prepared in accordance with the present invention are well known to be valuable intermediates in the manufacture of various dyes, agricultural pesticides, pharmeceutical and industrial compounds. For example, o-bromofluorobenzene may be converted to 3-fluorosalicylaldehyde for use in preparing oxygen absorbing solid state chelates such as "Fluomine" [cobalt-bis (3-fluorosalicylaldehyde) ethylenediimine].
The classical method for preparing aromatic fluorine compounds from aromatic amines is commonly referred to as the Schiemann method. This method is set forth in detail by Arthur Roe in Organic Reactions Vol. 5, Chapter 4, J. Wiley and Sons, Inc., New York (1949). In the Schiemann reaction the amine is diazotized to the corresponding diazonium fluoroborate. The diazonium fluoroborate is then isolated, purified and thermally decomposed. While quite satisfactory for the preparation of some compounds, the isolation, purification and particularly the thermal decomposition of many fluoroborates may be extremely hazardous due to a tendency to violently decompose. Methods have been devised to prevent violent decomposition but such methods involve a multiplicity of steps making the procedure an expensive one to practice.
A simpler, less dangerous method of converting aromatic amines to aryl fluorides is described in German Pat. No. 600,706. This method contemplates diazotization of the primary amine with sodium nitrite in anhydrous hydrogen fluoride. A solution is made of the primary amine and hydrogen fluoride and sodium nitrite is added to the mixture while temperature is maintained at about 5.degree.C. Upon completion of diazotization the reaction mixture is refluxed at a temperature of about 30.degree.-40.degree.C. whereupon the diazonium fluoride decomposes to nitrogen and the corresponding aromatic fluoride. The principle disadvantage of this method is that it is not useful commercially preparing aryl fluorides which are substituted in the ortho position with groups which contain one or more atoms with unshared electron pairs. There are several reasons for this. First, the corresponding diazonium fluorides are generally stable at these temperatures leading to negligible or inadequate yields. The use of higher temperatures and pressures has been found to lead to the formation of a complex mixture of high melting solids, again producing inadequate yields. See Ferm, R. L. and Vander Werf, C. A., J. Am. Chem. Soc., 72, 4809 (1950).
Anello et al., U.S. Pat. No. 3,160,623, teaches, with respect to derivatives other than the present ortho substituted derivatives, that decomposition of a diazonium fluoride prepared in the presence of a nitrosyl fluoride complex can advantageously be decomposed by heating the reaction mass to a temperature as high as 70.degree.C. which is also high enough to effect evolution of nitrogen. According to this teaching yields are substantially decreased when temperatures above 70.degree.C. are employed. Such temperatures, however, are inadequate to obtain satisfactory yields of the more stable ortho derivatives of the present invention.
It has now been discovered that by modifying thermal decomposition conditions these ortho substituted diazonium fluorides can be decomposed to provide high yields of corresponding ortho substituted aryl fluorides without forming excessive quantities of high melting solids.